The invention relates to a monolithic exhaust gas catalyst, or catalytic converter, which is arranged in a metallic housing and is flexibly supported against the inner surfaces of the housing by means of a resilient fiber material.
Known exhaust gas catalysts generally consist of a monolithic carrier which has a honeycomb-like structure and is provided with a catalytically active coating. The exhaust gas flows through the channels formed by the honeycomb and is thereby catalytically converted. Because both the metal housing and the exhaust gas catalyst are exposed to high temperatures, and are thus subject to thermal expansion, fixing the exhaust gas catalyst in the housing is not entirely simple. To compensate for the variable thermal expansions, a layer of resilient fiber material is usually arranged between the catalyst block and the housing. The fiber material accommodates the different thermal expansions and at the same time protects the catalyst block from the effects of shock, as described in, for example, German Unexamined Published Application (Offenlegungsschrift) No. 2,935,470.
It has been found that, in spite of the fact that sealing of the intermediate space between the exhaust gas catalyst block and the metallic housing has been constantly improved, with increasing operating time exhaust gas flows past the catalyst and through the layer of the resilient fiber material, thereby increasing the content of unconverted compounds in the exhaust of the vehicle.
It is thus an object of the invention to prevent, to the extent possible, the passage of untreated exhaust gases from the exhaust gas stream upstream of the catalyst into the exhaust gas stream downstream of the catalyst.
This object is achieved by providing the resilient fiber supporting the monolitic exhaust gas catalyst with a catalytically active coating.
The resilient fiber material generally employed is a coil of woven wire cloth, knitted wire cloth or a moulding produced from a network-like metal structure consisting of thin wires or of a metal felt. Furthermore, in order to obtain better thermal insulation, the resilient fiber material can be mixed with an inorganic fiber material, such as asbestos, or the said inorganic fiber material can run through the resilient fiber material (as disclosed in German Unexamined Published Application No. 2,935,470 or German Unexamined Published Application No. 2,920,604).
According to an embodiment of the invention, the fiber material is provided with a catalytically active coating by which the exhaust gases entering the fiber material are converted. The catalytically active coating can be applied onto the fiber material by the generally customary methods. The catalytically active substances used are the substances which are also employed in the catalyst element, for example the noble metals of the platinum group.
Because of the very low flow velocity of the exhaust gases through the resilient fiber material, very good chemical conversion of the exhaust gas takes place even at the lower temperatures close to the housing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.